The present invention relates to a magnetic clutch to be interposed between winding and rewinding members and a driving source in a device for winding and rewinding a wire or other material such as tape or paper.
Generally, any tension may be exerted on a wire or a medium such as tape or paper during the winding and rewinding of the same. In order to prevent the wire or the medium from braking under high tension and to prevent any overload from being applied to a driving source as the tension changes owing to changes in the radius of the wound or unwound winding, a magnetic clutch is arranged between the winding and rewinding members and the source.
A conventional magnetic clutch is shown in FIGS. 6 and 7. In FIG. 6, reference numeral 21 denotes a rotary shaft, 23 a hysteresis member, 22 a driven-side housing for holding the hysteresis member 23 and fixed to the shaft 21, 24 a magnetic substance member, 25 a driving-side housing for holding the magnetic substance member 24, 26 a bearing for separating the rotating operations of the driving-side housing 25 and the shaft 21, and 28 a spacer for maintaining a gap between the hysteresis member 23 and the magnetic substance member 24.
When the rotary power is transmitted from a driving source (not shown) to a pulley 27 integrated with the driving-side housing 25, the transfer torque T.sub.0 is applied from the driving-side housing 25 to the driven-side housing 22. The transfer torque T.sub.0 is determined by the material and volume of the hysteresis member 23 and of the magnetic substance member 24 and by the distance of the gap therebetween. As shown in FIG. 7, the torque is constant, independent of the number of relative revolutions (n).
In the above structure, however, since the transfer torque is normally constant during the winding operation shown in FIG. 2, for example, the winding tension is large during the initial period of the operation because of the smaller radius of the winding, while the winding tension is small during the final period thereof because of the bigger radius of the winding. In this case, when the wire or the medium such as tape or paper is wound, a very large compressive force is applied in the radial direction to the initially wound part of the wire or the medium of the winding because of the tightening phenomenon, resulting in possible damage to the wire or the medium. On the other hand, the finally wound part of the wire or the medium of the winding is non-uniform because of the starring phenomenon. The starring phenomenon is a known phenomenon in which wrinkles are produced at the outer portion of the winding owing to the relatively low tension under which that part of the wire or medium has been wound to form the outer portion of the winding.